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Articulated Trilobite Ontogeny: Suggestions for a Methodological Standard

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Articulated Trilobite Ontogeny: Suggestions for a Methodological Standard Journal of Paleontology, 95(2), 2021, p. 298–304 Copyright © The Author(s), 2020. Published by Cambridge University Press on behalf of The Paleontological Society. This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited. 0022-3360/21/1937-2337 doi: 10.1017/jpa.2020.96 Articulated trilobite ontogeny: suggestions for a methodological standard Nigel C. Hughes,1,2 Jonathan M. Adrain,3 James D. Holmes,4 Paul S. Hong,5 Melanie J. Hopkins,6 Jin-Bo Hou,1 Alessandro Minelli,7 Tae-Yoon S. Park,8 John R. Paterson,9 Jin Peng,10 Mark Webster,11 Xi-Guang Zhang,12 Xing-Liang Zhang,13,14 and Giuseppe Fusco7 1Department of Earth and Planetary Sciences, University of California, Riverside CA 92521, USA <[email protected]>, <jhou006@ ucr.edu> 2Geological Studies Unit, Indian Statistical Institute, 203 B.T. Road, Kolkata, West Bengal, 700108, India 3Department of Earth and Environmental Sciences, 115 Trowbridge Hall, University of Iowa, Iowa City, Iowa, 52242, USA <jonathan- [email protected]> 4School of Biological Sciences, University of Adelaide, North Terrace, Adelaide, SA 5005, Australia <[email protected]> 5Korea Institute of Geoscience and Mineral Resources, 124 Gwahak-ro, Yuseong-gu, Daejeon, 34132, Republic of Korea <hongps@kigam. re.kr> 6Division of Paleontology (Invertebrates), American Museum of Natural History, Central Park West at 79th St., New York, NY 10024, USA <[email protected]> 7Department of Biology, University of Padova, via U. Bassi 58/B, 35131 Padova, Italy <[email protected]>, <giuseppe.fusco@ unipd.it> 8Division of Polar Earth-System Sciences, Korea Polar Research Institute, 26 Songdomirae-ro, Yeonsu-gu, Incheon, 21990, Republic of Korea <[email protected]> 9Palaeoscience Research Centre, School of Environmental and Rural Science, University of New England, Armidale, NSW 2351, Australia <[email protected]> 10College of Resource and Environment Engineering, Guizhou University, Guiyang, China <[email protected]> 11Department of the Geophysical Sciences, University of Chicago, 5734 South Ellis Avenue, Chicago IL 60637, USA <mwebster@geosci. uchicago.edu> 12Key Laboratory for Palaeobiology, Yunnan University, Kunming, 650091, China <[email protected]> 13Shaanxi Key Laboratory of Early Life and Environment, Department of Geology, Northwest University, Taibaibeilu 229, Xi’an 710069, China <[email protected]> 14Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, Nanjing 210008, China Abstract.—In order to maximize the utility of future studies of trilobite ontogeny, we propose a set of standard practices that relate to the collection, nomenclature, description, depiction, and interpretation of ontogenetic series inferred from articulated specimens belonging to individual species. In some cases, these suggestions may also apply to ontogenetic studies of other fossilized taxa. Introduction (Sánchez, 2012). As a result, interest in describing and interpret- ing ancient ontogenetic series has burgeoned. Recent years have seen progress in describing ancient ontogenies Investigations of fossilized ontogenetic series can be aided by in ways that can be compared with those of living taxa, even at the defining standard analytical practices, including evaluation of the level of patterns and mechanisms of developmental control. By limitations that fossilization places on our ability to interpret them. conducting morphometric analysis of appropriate data sets Here, we review concepts and procedures relating to the descrip- derived from fossils it is now possible to move beyond simply tion and interpretation of articulated trilobite ontogeny. Although describing sequences of ontogenetic stages, and to address ques- the focus of this contribution is on articulated trilobites (a general tions of high interest for evolutionary developmental biology. overview of trilobite ontogeny can be found in the legend of This provides insight into how developmental processes evolve Figure 1), many of these issues apply generally in studies of and how such processes affect the evolution of organismal ancient ontogeny, in particular in other arthropod or arthropod- body patterning. As such studies progress in number and taxo- related taxa. Our aim is to highlight methodological standards nomic scope, it is becoming possible to assess variation in devel- that may increase the comparative value of future studies. opmental trajectories among and across clades, and thus to move beyond the typological approach dictated by rare exemplars. It is Need for a standard approach increasingly apparent that even the ancient fossil record can reveal subtle patterns of microevolutionary-scale variation that has The formulation and application of a set of standard practices potential for insights into how aspects of body patterning evolved and minimal requirements in quantitative studies of ancient 298 Downloaded from https://www.cambridge.org/core. IP address: 170.106.202.226, on 01 Oct 2021 at 19:15:56, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/jpa.2020.96 Hughes et al.—Standard for trilobite ontogeny description 299 Downloaded from https://www.cambridge.org/core. IP address: 170.106.202.226, on 01 Oct 2021 at 19:15:56, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/jpa.2020.96 300 Journal of Paleontology 95(2):298–304 Figure 1. (1) Ontogenetic hypothesis of the alternative segmentation pathways of a polymorphic species across several postembryonic stages (s0–sn) or instars. Different morphological conditions (morphs, m), here based exclusively on the segmental pattern, are indicated as mXY, where X is the number of thoracic segments and Y the number of pygidial segments. Trunk segment addition initially proceeded at the pace of one or two segments per stage until stage s2, producing a poly- morphism in the number of trunk segments from stage s1 onwards. From stage s2 onwards, trunk segment addition proceeded at the pace of one segment per stage for all individuals, until the end of the anamorphic phase, at stage s4 for some specimens (m45) or at stage s5 for others (m44 and m43). Thoracic segment release pro- ceeded at the pace of one segment per stage and ended at stage s5 for all specimens. Morphs with five thoracic segments are holaspids (i.e., specimens that have reached a stable number of thoracic segments). Lines connect the succeeding morphological conditions of individuals across stages. The thickness of the lines con- necting the morphs reflects the frequency of the different morphs at each stage. (2) Misleading representation of the ontogenetic hypothesis in (1), where morphs (segmental condition) and stages are confused, and the former are seen as sequential meraspid and holaspid stages. Note the apparent reabsorption of one segment among some successive ‘stages.’ (3) One among several possible “correct” representations of the ontogenetic hypothesis in (1). This is correct only if the legend explains that there is variation in the pattern of segment addition within the sample and thus in the number of pygidial segments represented at several stages, and that the ontogenetic pattern depicted represents only one (possibly the more common) among alternative patterns. Parts (2) and (3) adopt a graphic style that has become common since Hughes et al. (2006), but other schematizations are possible. Colors highlight the three main body regions in trilobites: cephalon, blue; thorax (with dorsally articulated segments), light gray; pygidium (with conjoined segments), pink. The thorax and pygidium together form the trunk. During post-embryonic development, new segments were added subterminally, just in front of the terminal piece—triangle in (2) and (3). During the first post-embryonic stages (five stages shown in this schematic), the most anterior pygidial segment was progressively incorporated into the thorax region. This process, which consisted of the formation of a functional articulation in the posterior of that segment, is called “release.” Trilobite post-embryonic development is traditionally divided into three main periods: protaspid (not represented in the figure), meraspid (a period of segmental accretion in the thorax), and holaspid (a final period with a segmentally stable thorax). Alternatively, it can be divided into two phases: anamorphic (a phase of subterminal trunk segment addition) and epimorphic (a phase with a stable number of trunk segments). The presence of an anamorphic phase followed by an epimorphic phase qualifies trilobite development as hemianamorphic. In the specific case depicted, the onset of the epimorphic phase (termination of trunk segment addition) preceded the onset of the holaspid period (termination of thoracic segment release). However, this was not the case for all trilobite taxa. A more detailed account of trilobite ontogeny can be found in Hughes et al. (2006). ontogeny offers several potential benefits. Firstly, it may clarify (including section thickness) from which the specimens have the methodology of investigation in order to ensure that a com- been derived is of critical importance for inferring the span of mon descriptive framework is applied among different studies time over which specimens in the sample accumulated.
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